Are multiple balls coming out of the barrel when it only fires once, or is it firing multiple times?
The reason why I ask about multiple balls is that if the detents are bad, you can fire more than one ball at a time. But usually the second ball doesn't travel far or breaks in the barrel with the next shot.
If it is firing multiple times when you pull the trigger once, You may be getting trigger bounce. Check to make sure the selonoid (under the grip) isn't getting squeezed by the grips. My nephew has a Pilot that would fire multiple times when he squeezed the grips tightly against the 'noid.
Finally, give it a good cleaning and oiling.